1. Field of the Invention
The present invention relates to a technique for detecting stains on a printed product.
2. Description of the Related Art
In a printer that prints a sheet by ejecting ink, ink may have inadvertently dripped from an ink ejection print head, and be attached to the sheet, or may be attached to a path in which the sheet is to be conveyed, thereby causing ink stains on the sheet. Therefore, printers that include a stain detection function and a recovery function are well known.
In Japanese Patent Laid-Open No. H03-197140 (1991), a printing apparatus equipped with a stain detection function is disclosed. The printing apparatus disclosed in the publication employs brightness information for a blank margin of a printed matter to detect the presence of stains. Specifically, the normal brightness of the blank margin is stored as a reference value, and each time the blank margin is detected, the brightness of the detected blank margin is compared with the normal brightness of the blank margin, and when a difference of the two exceeds a predetermined threshold value, it is ascertained that stains are present.
According to the printing apparatus disclosed in Japanese Patent Laid-Open No. H03-197140 (1991), the brightness information of a blank margin is employed to detect ink stains, which have occurred due to unexpected ejection of ink because of the defect of the apparatus, and stains that are originally attached to a printed product. However, the printing apparatus disclosed in Japanese Patent Laid-Open No. H03-197140 (1991) detects stains that have appeared in the blank margin, but can not detect stains in the other portions. That is, the stain occurred in the image can not be detected. Furthermore, the printing apparatus disclosed in Japanese Patent Laid-Open No. 03-197140 (1991) can determine only the presence of stains in the margin, but can not identify the type of stain, and therefore, a cause of the stain can not be specified based on the detection results.
Moreover, according to Japanese Patent Laid-Open No. H03-197140 (1991), after stains in the blank margin have been detected, the printing apparatus does not control a recovery process based on the detection results. Generally, in a case wherein stains that appear in a printed matter are those caused by the ink ejection unit of the printing apparatus, it is required that an appropriate recovery process be selected and performed from among a plurality of recovery processes for recovering the ejection function of the ink ejection unit. However, as described above, according to the printing apparatus disclosed in Japanese Patent Laid-Open No. H03-197140 (1991), since a user selects a recovery process to perform, there is a possibility that an incorrect recovery process may be performed. When the incorrect recovery process is performed, the condition would become worse, e.g., the range of the stain would be extended.